This invention relates to the operation of a poly-phase DC motor, and more particularly, to a method and a circuit for rapidly and reliably switching from pulse width modulation mode to linear control mode for operation of a poly-phase DC motor.
Poly-phase DC motors, and more particularly three-phase DC motors of the brush-less, sensor-less type, are widely used in computer system disk drives, such as floppy disk, hard disk, or CD ROM drives, as well as in other applications. Such motors can be driven in two different control modes: pulse width modulation or linear.
In many systems, there is need to switch between pulse-width modulation control mode to linear control mode. In some applications, this switching is done once per revolution of the motor. In other applications, the control mode is switched several times per revolution. In still other applications, the motor is primarily driven in linear mode and occasionally in pulse width modulation mode. Frequently, this switching occurs at a selected time in the revolution of the motor. Often, the selected time is when the voltage induced in the coil that is floating, i.e., the back EMF voltage, exhibits a zero crossing point.
One problem encountered with switching from PWM control mode to linear control mode is that linear control circuitry requires a finite time to stabilize after switching to achieve reliable operation. This settling period is in part because the bias circuit of the linear control circuitry tends to partially discharge while the controller is operating in pulse width modulation mode, and requires time to be recharged to an appropriate voltage.
In a first embodiment, the present invention includes a circuit having a differential input amplifier having a first input responsive to a first voltage, a second input responsive to a second voltage and having an output coupled to a first node. The circuit also includes a transconductance operational amplifier having an input responsive to voltage signals coupled to the node and an output providing a voltage signal. A bias circuit is coupled from a power supply lead to disable the differential input amplifier and the transconductance operational amplifier and providing a high DC impedance at the first node in response to a control signal.
A drive circuit provides power to a motor in a linear mode. At a selected time, the drive circuit transitions from the linear mode of operation to a pulse width modulation mode of operation for providing power to the motor. When the circuit is in the pulse width modulation mode, the bias current terminal for the linear drive circuit is disconnected from all discharge paths to prevent a capacitor connected to this bias terminal from discharging. The voltage on the bias terminal is maintained even though the linear drive circuit currently is not being provided with power and is not operating. At a selected time period, the circuit transitions from the PWM mode of operation back to the linear mode of operation. The voltage bias terminal has maintained the same voltage which was present when it was disabled because the node has been disconnected from all discharge paths and placed into a high impedance state. This provides the advantage that transients associated with charging and discharging of this node are avoided, and fast switching from linear mode to PWM mode and back to linear mode is possible. Further, noises associated with such switching are reduced by maintaining the bias circuit disconnected from all discharge paths when the circuit is not operating.
The invention provides a linear mode control circuit and accompanying method for promoting rapid and robust linear mode control in response to control signals for powering DC poly-phase brush-less motors. The invention also provides a circuit and an accompanying method for maintaining the charge on the bias circuit while the motor is being driven in the PWM mode.